This invention relates generally to electronic particle counting and analyzing apparatus utilizing an aperture and, in particular, relates to correcting errors in counting and analyzing resulting from the coincidence of a plurality of particles in the aperture.
Particle analysis apparatus using an aperture is well-known. See U.S. Pat. No. 2,656,508. The aperture of such apparatus provides a minute scanning aperture, ambit, or sensing zone relative to or through which pass and are detected, counted, and analyzed, single particles at a rate often well in excess of one thousand per second. Coincidence of two particles in the scanning ambit occurs quite often due to the physical parameters of the scanning aperture and particle concentration. The effect is that one particle is believed to be detected, counted and analyzed while two or more particles are actually located simultaneously in the sensing zone.
Correction for counting errors caused by such coincidence has been performed in several ways. One way is for an operator to refer to a coincidence correction chart which presents the proper error corrected count for a very large selection of counts produced by the device. This provides an accurate result but is time consuming and prohibits fully automatic recording and processing of error corrected counts.
Other ways have been developed which electrically manipulate the acutal count or the manner of obtaining the actual count to provide an error corrected count. U.S. Pat. No. 3,626,164 (U.S. Class 235/151.3) discloses circuitry which adds counts to the detected count to yield a corrected count which closely approximates the true particle count. U.S. Pat. No. 3,936,740 (U.S. Class 324/71 CP) discloses circuitry which digitally delays producing the pulses received from the Coulter aperture. U.S. Pat. No. 3,949,197 (U.S. Class 235/92 PC) discloses circuitry which provides a statistical correction to a detected train of particle derived count pulses so that the effective random coincidence loss or gain of the count does not induce ultimate counting error. And, U.S. Pat. No. 4,009,443 (U.S. Class 328/11) discloses circuitry which alters the time period in which particle pulses are counted. Other examples of coincidence correction in a Coulter aperture environment exist, but the stated examples are believed to be indicative of the cited U.S. classifications.
Thus, until the present invention, coincidence correction of a count of particles obtained in an aperture environment has been by operation on the electrical signals obtained from the aperture based upon some statistical theory. No attempt has been made directly to determine when this error causing coincidence exists.
In addition to counting errors, coincidence causes errors in analyzing other parameters, such as particle volume. When a particle passes through an aperture, its volume may be analyzed by the amount of change in electrical current flow through the aperture. Coincidence of two particles in the aperture causes a change in the flow of electrical current different from the change effected by either particle individually. This causes errors in particle volume determination which must be corrected for to determine the true particle volume. Again, no attempt previously has been made directly to determine when this error causing coincidence exists.
The present invention provides a method and apparatus for directly determining when a coincidence of particles in an aperture exists. The results of this direct determination may be used as desired, such as by correcting data containing coincidence errors, determining when valid data exists, or otherwise.